The present invention relates to a reflecting prism such as a double Porro prism for changing a direction of propagation and/or an orientation of an image of an object. More specifically, the present invention relates to a reflecting prism for erecting an image used in a finder system of a camera.
A real image finder of a camera requires an erecting system for erecting an orientation of an image formed by an objective lens system. A double Porro prism is used in a conventional real image finder.
FIG. 8 shows a conventional double Porro prism 3. A light ray L1 is incident on a surface 3a and internally reflected by first, second, third and fourth reflecting surfaces R1, R2, R3 and R4 and exits the double Porro prism 3 and a surface 3b.
FIG. 9 shows a real image finder having the conventional double Porro prism 3. The double Porro Prism 3 is shown "unfolded" (i.e., transformed such that the optical axis is shown as a straight line). The light rays from an object are focused by an objective lens 1 on a screen 2. The image on the screen 2 is erected by the double Porro prism 3, and a viewed by a user through an eyepiece lens 4.
The surfaces 3a and 3b are coated with an anti-reflection coating, while the four reflecting surfaces R1 through R4 are coated so as to improve their reflectivity. Other surfaces such as an "unused surface" 3c are formed as diffusing surfaces. The term "unused surface" in this specification refers to surfaces which are not used for transmission of light incident on the prism (i.e., light L1).
In order to make the finder compact, the erecting system should be as small as possible. However, when the small erecting system is used, a width d of an optical path of the double Porro prism 3 becomes narrow. With this construction, a small amount of light (corresponding to a peripheral part of an image and shown as a dotted light ray in FIGS. 9 and 10) may be totally internally reflected by the surface 3b to be incident on the reflecting surface R4 of the prism 3. In FIG. 9, the surface 3b is mapped onto surfaces 3b' and 3b". The surface 3b' represents the surface 3b when light passes through it, whereas the surface 3b" represents the surface 3b when light is totally internally reflected by the surface 3b.
Therefore, when the small amount of light is reflected by the surface 3b (shown as surface 3b" in FIG. 9) it is reflected by the surface R4 and then transmitted through the surface 3b (shown as surface 3b' in FIG. 9) to the eyepiece lens 4. This results in ghost reflections occurring which reduce the clarity of the image seen through the eyepiece lens 4.